This invention relates generally to optical scatterometers and more specifically to an improved optical scatterometer exhibiting improved sensitivity and bandwidth characteristics. An optical scatterometer can be employed in a number of manufacturing environments that require knowledge of surface microstructure. Since the scatterometer technique is a noncontact diagnostic technique, it does not result in damage to the sample under investigation. In addition, the noncontact nature of the technique makes it more suited for production monitoring procedures. For example, for over a decade the optical scatterometer has been used to examine surfaces of optical components, such as dielectric mirrors, metal mirrors, and glass substrates. Similarly, finely machined components can be monitored with an optical scatterometer during manufacture. The present invention relates to an optical scatterometer useful for examining the microstructure of smooth optical components, magnetic and optical storage media, and semiconductor microelectronics components during fabrication. By monitoring the size distribution of microstructure of semiconductor silicon substrates and the film subsequently deposited on the substrates one can obtain quantitative information concerning component microstructure. This knowledge can then be employed in the manufacturing process to improve performance, yield, and lifetime of semiconductor products.
In operation of a scatterometer, the sample under examination is illumninated with light of visible or infrared wavelengths, and the light which is scattered by the sample is analyzed, as shown in the prior art scatterometer arrangement of FIG. 1. In the case of some samples, such as optical components, for example, knowledge of the scattered light is itself important. In the case of other samples, the fact that a surface scatters light might not be of direct importance. However, the scattered light can be analyzed to suggest characteristics of the microstructure of the sample under analysis. In the case of surfaces which do not transmit light at the wavelength of use in the scatterometer, the scattered light is indicative of the surface microstructure of the sample. Metallic surfaces are an example (e.g. Ag surface examined with light of wavelength 633 nm.) If the sample transmits light at the wavelength of use in the scatterometer, the scattered light is indicative of both surface and volume microstructure of the sample. An example of this situation is a thin film of Si.sub.3 N.sub.4 on a silicon substrate being examined with light of wavelength 633 nm. The use of two light sources in one scatterometer includes both situations mentioned above, thus extending the current capabilities of the scatterometer analysis technique.
As used herein, the term microstructure refers to surface microroughness and the way in which the microroughness is distributed at different spatial frequencies, or differenct sizes, of structure on the surface of interest. This is to be distinguished from isolated surface defects and contaminants such as dust particles. However, the scatterometer described herein is useful for detecting and quantifying these on a surface as well.
There are two key requirements of an optical scatterometer. First, the scatterometer must be sufficiently sensitive to detect scatter from surfaces of the minimum microroughness of interest. Second, the scatterometer must be capable of detecting scatter from microstructure within the range of spatial frequencies, or sizes, of interest. For example, microelectronics processing requirements include minimizing the amount of microstructure as short as 0.5 micron (.mu.) lateral dimension (2 inverse micron (.mu..sup.-1) spatial frequency) on a surface. Hence, the scatterometer must be capable of detecting scattered light from structure of this size. The relation between microstructure size and scatterometer parameters is discussed below. These requirements are difficult to achieve, expecially when samples to be examined are very smooth, and therefore have low scatter. The present invention extends the capabilities of existing scatterometers in satisfying both requirements.